Electroplating of aluminum

ABSTRACT

ALUMINUM IS ELECTROPLATED UPON METALLIC SUBSTRATES BY IMMERSING THE SUBSTRATE AS A CATHODE IN AN ELECTOYTE WHICH IS AN ELECTRICALLY CONDUCTIVE ORGANOALUMINUM COMPLEX. AN ALKYL HALIDE IS INCLUDED IN THE ELECTROLYTE IN AN AMOUNT OF FROM 1 TO 10 WEIGHT PERCENT AND FUNCTIONS AS A DEPOSIT MODIFIER.

United States Patent O 3,672,965 ELECTROPLATING OF ALUMINUM William H. Harwood, Lawton, Okla., assignor to Continental Oil Company, Ponca City, Okla. No Drawing. Filed June 29, 1970, Ser. No. 50,883 Int. Cl. C23b /00 US. Cl. 204-14 N Claims ABSTRACT OF THE DISCLOSURE Aluminum is electroplated upon metallic substrates by immersing the substrate as a cathode in an electrolyte which is an electrically conductive organoaluminum complex. An alkyl halide is included in the electrolyte in an amount of from 1 to 10 weight percent and functions as a deposit modifier.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to the electroplating of aluminum on metallic substrates.

Brief description of the prior art It is well known that it is difiicult to efficiently and economically electroplate aluminum on metallic substrates. Various electrolytes and deposit modifiers have previously been proposed for this purpose in an aifort to reduce the temperatures and current consumption required, and to increase the current efiiciency. Among the most well known electrolyte systems are aluminum organometallic complex compounds used in conjunction with onganoaluminum compounds; solutions of an aluminum salt in an organic solvent, such as ethyl ether; organic complexes of aluminum halohydrides; molten salt systems containing anhydrous aluminum halides; an alkali metal chloride and an alkali metal fluoride; and baths consisting essentially of an aluminum halide and the reaction product of benzene with an alkyl halide. All of these systems have still failed to provide optimum electroplating due to high current density requirements, the occlusion of undesirable impurities in the coating obtained, safety hazards inherent in the electrolyte, mossiness or treeing in the coating of aluminum plated on the substrate, or other undesirable aspects.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The present invention provides an improved method for electroplating a metallic substrate with aluminum in which an electrically conductive organoaluminum complex is utilized as the electrolyte, and an alkyl halide compound is used as a deposit modifier. Various types of organoaluminum complexes can be utilized, and the alkyl halide compound utilized as a deposit modifier may be any compound of the formula RX, where R is an alkyl radical and X is selected from the group consisting of chloride, bromine, and iodine. The deposit modifier is included in the electrolyte system in an amount of from about 1 weight percent to about 10 weight percent.

The electrolyte is maintained at a temperature of from about 0 C. to about 100 C. during the electroplating of the substrate, and current densities of from about 1 ma./cm. to about 1 ampere/cm. can be utilized. A high purity aluminum anode is preferably used.

An object of the invention is to provide a smooth, shiny aluminum plating on a metallic substrate by an electroplating process.

A more specific object of the invention is to provide a deposit modifier for use with organoaluminum complex 3,672,965 Patented June 27, 1972 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION The metals plated with aluminum by the process of this invention can be substantially any dissimilar metal such as steel, copper, iron, lead, tin, zinc, brass or bronze. The metal is shaped to correspond to an article which it is desired to aluminum plate, and is made the cathode positioned within the cell for immersion in the electrolyte bath. The anode employed consists of aluminum which is preferably of relatively high purity to avoid undesirable alloying in the plate which is deposited, unless such alloying should be desired, in which event an aluminum alloy can be usefully employed as the anode.

The electrolyte bath used in the invention consists essentially of an electrically conductive organoaluminum complex containing from about 1 weight percent to about 10 weight percent, based on the total weight of the electrolyte system, of a deposit modifier of the formula RX, where R is an alkyl radical, and X is selected from the group consisting of chloride, bromine and iodine.

Various electrically conductive organoaluminum complexes are useful in the process of the invention, and the utility of many of these as major components of aluminum plating baths has been heretofore recognized. Examples of organoaluminum complexes which are electrically conductive and thus suitable for use in the invention are lower aluminum trialkyl metallic complexes of the formula MeX-2AlR where Me is a metal-preferably an alkali metal, X is halogen, and R is a lower alkyl radical containing from 1 to 5 carbon atoms; lower aluminum dialkyl metallic complexes of the formula MeX-AlR X', where Me, X and R have the meanings set forth above, and X is halogen which may be identical to, or different from, X; and complexes of the formula MeAlR X where Me, R and X have the meanings set forth above, and x is an integer of from 0 to, and including, 4. Etherate complexes of organoaluminum compounds are also electrically conductive and suitable. In preparing such etherate complexes, a wide variety of others may be utilized, such as ethyl ether, propyl ether, butyl ether, octyl ether, etc.

Of the types of organoaluminum complexes useful in the invention, the alkali metal aluminum alkyl halide complexes are preferred. These compounds have the formula MeAlR X as defined above. Sodium aluminum ethyl trichloride functions especially well in conjunction with the alkyl halide deposit modifiers hereinafter described in greater detail.

The deposit modifier utilized in the electrolyte system is selected from the group consisting of alkyl chlorides, alkyl bromides and alkyl iodides. The alkyl radical, though not so limited, is preferably of straight chain configuration, and preferably contains from 2 to 8 carbon atoms. Examples of useful deposit modifiers include ethyl chloride, propyl bromide, octyl iodide, iso-butyl chloride, and tertiary amyl bromide. The modifier constitutes from about 1 wieght percent to about 10 weight percent of the total weight of the electrolyte system, and preferably from about 1 weight percent to about 5 3 weight percent is employed, and is distributed evenly throughout the organoaluminum complex.

In some circumstances, such as where it is desirable to reduce the melting point of the bath, or to reduce its 4 fied in Table I, are set forth in Table II. It will be noted that where the electrolyte systems of the present invention were employed, very smooth plating was obtained as contrasted with the rough, mossy plating obtained when viscosity, it may be desirable to add an inert diluent to 5 the deposit modifier of the present invention was omitted the electrolyte. Typical materials which can be employed from the system. for this purpose include aromatic hydrocarbons, tetrahy- TABLE H drofurane and various ethers such as diphenyl ether and diethyl ether. 6,13 dichlorethyl ether may be used in fgfigtfigfi f l small amounts to improve the smoothness and hardness 0 un pos Clancy, Remarks on plating of the plating. percent The electroplating bath is maintained at temperatures X 82 Very ssywhich are, in general, above that required to maintain 323%,}? 3% i b the electrolyte in a molten, or relatively low viscosity x 98 3 state (where a diluent is used). The upper temperature 5 241x10 96 limit is dictated by the decomposition temperature of the Although certain preferred embodiments of the invem complex. I have found that temperatures in the range of d d d t 0 e m from about to can be 3f; Enginestsz iivzmiasnz at; utilized with most systems, and operation at room tem- 3- perature offers economic advantages in a number of inthat vanatlms m p gi i g i f stances. Since the electrical conductivity of the elecamounts 0 the vanous lens 0 as s i trolytes increases sharply with temperature, it is often i t can be emPloyed.wlt out departure.mm aslc desirable to use bath temperatures of about 100 C. or prlmmples of the mventlon' Changes mnovailons of above where the specific conductivity of the complex in W are therefore detained be cucumscgged by use is relatively low. the spirit and scope of the invention except as t y may As is well understood in the art, it is desirable to probe necessanly .hmlted by the accompanymg claims or tect the heated melts of the organoaluminum complex reasnab le ecimvalnts thereof electrolytes from contact with air. This may be acwhat is claimed is: complished by covering them with a small amount of Process for eieeti'opiatlng metals with aluminum parafiin oil, but is preferably accomplished by blanketing 0 compilslugi the bath with an inert gas, such as nitrogen. The elecprovldmg the metal to be pl d as a cathode, and troplating bath should also be prepared and maintained Providing an anode eousistlug essentially of aiumi under anhydrous diti num metal, in an electrolyte consisting essentially of The current densities and voltages employed are, of an elecmcfliiy conductlv? organoalummum complex course, widely variable, and their selection will be i containmg afiecnye amount of an alkyl dependent upon the electrolyte used, the amount, if any, deposlt having theflformula Where of diluent used, the temperature of the bath, the electrode R is an f i radlcal X 15 selected q spacing and the specific type of plating desired. These grfmp conilstmg of q bromme Iodine considerations and their effect upon voltage and current said elefimcauy conductwe t, density requirements are well understood in the art. The 40 Flex bemg seleqed from the group consisting of current densities utilized should, in general, however, cmpunds,havmg the formula Mex'zAlks range from about 1 ma./cm. to about I ampere/0m Pounds having the formula MBX'AIRZX and It may also be stated as a general proposition that it is Pounds hav1ng,th formula M where desirable, in most instances, to use the highest current is a m t X halogen R is a lower alkyl radical densities possible for economic reasons. Fontammg from 1 to 5 carbon Fltoms, and x is an The following working examples will serve to further Integer of from! 0 and including 4; illustrate the process of the invention. In the examples, Passing an elFcmc current between the electrodes to a number of electroplating runs were carried out using Piate aluminum upon the Q various electrolyte systems within the scope of the A Process as defined in claim 1 wherein said aikyi present invention for accomplishing the deposition of halide deposit modifier is Present in the electrolyte in an aluminum metal on iron and platinum electrodes. The amount of from about 1 Weight Percent to about 10 Weight composition of several electrolyte systems used, and the Percent of the total Weight of the electrolyte y electrical parameters employed, are set forth in Table 3 A Process as defined in claim 1 wherein said bath I. It will be noted that the first run is a control run since is maintained at a temperature of from about to the system did not include any deposit modifier in the about dul'iug the Passage of electric current electrolyte system. In each one of the runs, a Pyrex glass tween said electrodes. cell having a volume of 50 ml. was utilized, and a Aprocess as definedin claim 1 wherein said organemedium-grade, glass-fritted disk was used for separating aluminum eomPieX has the formula x (4x) the anode compartment from the cathode compartment. Where e is an alkali metal, R is a loWer aikyl'radieal The distance of separation of the electrodes was 2 inches. t ing fro 1 t0 5 carbon atoms, X. is halogen, and In Runs 3, 4, and 5 a platinum cathode was employed, x is an intege of from 0 and including, and in Runs 1 and 2, an iron cathode was utilized. An 5- A p oc ss as definedin Claim 1 wherein Said alkyl aluminum anode was utilized in all of the runs. radical contains from 2 to 8 carbon atoms.

TABLE I Bath Complex Deposit modifier tempera- Current Weight Weight tux-e, Voltdensity, Run percent percent C. age maJcm e 100 NaAKCzHr) on None 55 12 s 90 (C2H5) C13 10 021150] 55 12 6 90 (CQHQMO .Al(C2H5)Ch 1O CgH5Cl 23 12 4 96 (C2Hs) C12 4 CzHsCl 0 510 10 AKCZHS) 01 30 CQHSCI 0 14 100 The results obtained with the several electrolyte sys- 6. A process as defined in claim 1 wherein the current terns, and with the imposed electrical conditions identidensity of the electric current passed between the electrodes is from about 1 ma./cm. to about 1 ampere/cm. at the cathode surface.

7. A process as defined in claim 2 wherein said deposit modifier is present in the electrolyte in an amount of from about 1 weight percent to about 5 weight percent of the total weight of the electrolyte system.

8. A process as defined in claim 3 wherein said alkyl halide modifier is present in the electrolyte in an amount of from about 1 weight percent to about 10 weight percent of the total weight of the electrolyte system.

9. A process as defined in claim 4 wherein said organealnminum complex is sodium aluminum ethyl trichloride.

10. A process as defined in claim 8 wherein said organoaluminum complex has the formula MeAlR X where Me is an alkali metal, R is a lower alkyl radical containing from 1 to 5 carbon atoms, X is halogen, and x is an integer of from 0 to, and including, 4.

6 References Cited UNITED STATES PATENTS Electrodeposition of Aluminum from Non-aqueous Solutions, The Electrochemical Society Preprint 65-2,

10 Aprll 1934, pp. 25-37.

JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner US. Cl. X.R.

8/1939 Mathers et a1 20414 N c 

